Here are a few pointers to get you started.
Completely unofficial/unsupported, so use at your own risk (and be super careful, unplug everything before making/breaking electrical connections)
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I strongly advise to initially NOT connect the signal from the Shapeoko. Use the VFD factory settings, adjust parameters to match your spindle characteristics as per the provided documentation, and check if you can control the spindle manually from the VFD, and if it spins in the correct direction.
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once the spindle operates correctly in manual mode, you can enable external control to use the PWM signal from the Shapeoko controller to drive the RPM. This should boil down to:
- pick-up the PWM signal on one of the controller’s connector (typically the BitRunner connector). The pinout is out there on the forum.
- connect GND to your VFD’s GND, and PWM to the “5V Input” on your VFD.
- set Pn 03 to 3
- set Pn 04 to 2
Those two should make the spindle controllable by the Shapeoko controller, and then you still have to modify GRBL’s param $30 to 24000, such that M3/M5 gcode commands are correctly translated to 0-5V output on the PWM.
There are quite a few VFD-related threads on the forum (this long one for example), but details depend on your setup/VFD/spindle.